It is necessary or desirable to monitor location of certain people. For example, it may be desirable to monitor the location of a mentally ill person or criminal, particularly whether such a person enters or leaves a particular zone. Inputting a zone using a graphical user interface interacting with a mapping overlay system on a website and then transmitting data defining the zone to a device worn by a person to be monitored is known. In this case the device may send an alert if the person enters or exits that zone. However, defining a zone in such a way is difficult and the boundaries of the zone as defined may not reflect where the boundaries are in actuality. In addition, defining the zone manually is time-consuming. It is an object of the present invention to address these difficulties.